This invention concerns radiation therapy, especially brachytherapy, for treating tissues which may have diffuse proliferative disease.
In brachytherapy, the radiation source is generally placed within a surgically created or naturally occurring cavity in the body. In particular, this invention relates to delivery of radiation therapy to tissue as might be found in the human breast, or to other tissue, often following surgical treatment of cancer.
Radiation therapy following tumor resection or partial resection is generally administered over a period of time in partial doses, or fractions, the sum of which comprises a total prescribed dose. This fractional application takes advantage of cell recovery differences between normal and cancerous tissue whereby normal tissue tends to recover between fractions, while cancerous tissue tends not to recover.
In brachytherapy, a prescribed dose is selected by the therapist to be administered to a volume of tissue (the target tissue) lying outside the treatment cavity into which the radiation source will be placed. Generally the prescribed dose will include a minimum dose to be delivered at a preferred depth outside the treatment cavity (the prescription depth or point). Since, in accordance with the laws of physics, radiation intensity falls off sharply with increasing distance from the radiation source, especially from the very near regions outwardly, it is desirable to create and maintain a space between the source of radiation and the first tissue surface to be treated (generally the cavity wall since the source is placed within the cavity) in order to moderate the absorbed dose at the cavity surface. This is often done by placing a balloon or other applicator in the cavity with the radiation source inside the applicator. Such balloons are preferably inflated with a fluid which is substantially water and since water is similar to soft tissue with respect to radiation attenuation, this simplifies prescription planning.
Often the prescription depth outside the cavity is to be uniform. In this isotropic case, it is therefore important that the incident radiation on the interior surface of the cavity be the same at all points being treated. Depending on the emission pattern of the source being used, it may be necessary to sequentially position a single radiation source through a series of positions, often within a source channel or guide tube positioned on an axis of symmetry of the applicator balloon (or utilize other positions or multiple sources strategically placed) to produce the desired uniformity in the aggregate. Furthermore, by selecting the radiation source intensity (radioisotope emissions or x-ray tube output) and controlling treatment time and the distance from the source(s) to the cavity interior surface, the incident radiation can be sufficiently moderated to avoid substantial damage to normal tissue.
In contrast to the isotropic situation just described, the treatment cavity may be near sensitive tissue structures, e.g., skin, such that an isotropic prescription plan may include points which intersect or encompass such structures. In such a situation, the therapist may be forced to locally shield emitted radiation from within the treatment cavity (see co-pending application Ser. No. 11/471,277 incorporated by reference herein in its entirety), or to resort to radiation sources which emit anisotropically. Such capabilities may not be readily available or practical. In such cases, and there are many, the patient may therefore be denied the advantages of brachytherapy.
One accepted standard in radiation therapy for the range of applicators currently in use is that a one centimeter prescription depth of tissue outside the treatment cavity be used for dose planning. Assuming the tissue at the prescription depth receives the desired dose, the tissue nearest the source should not receive more than 2.5 to 3 times the prescription dose. Standards also usually require that the skin not receive a dose of more than 1.5 times the prescription dose. With a one centimeter prescription depth, the balloon diameter must be about 2.7 to 3.4 cm diameter to meet the above near-tissue standard, and this usually requires the skin be at least 6-8 mm out from the surface of a balloon applicator engaged against the tissue in a cavity. A distance of less than about 6-8 mm may result in doses higher than 1.5 times the prescription dose which are known often to result in undesirable cosmesis. This problem commonly arises after a lumpectomy and is a counter-indication for isotropic brachytherapy. In order to make brachytherapy available to more patients having resection cavities in close proximity to skin surfaces or to other radiation sensitive structures, the apparatus and/or methods of this invention may be employed.